Automobiles and trucks receive exterior finishes for several well known reasons. First, such finishes provide barrier protection against corrosion. Second, consumers prefer an exterior finish having an attractive aesthetic finish, including high gloss and excellent DOI (distinctness of image).
A typical automobile steel panel or substrate has several layers of finishes or coatings. The substrate is typically first coated with an inorganic rust-proofing zinc or iron phosphate layer over which is provided a primer which can be an electrocoated primer or a repair primer. Optionally, a primer surfacer can be applied to provide for better appearance and/or improved adhesion. A pigmented basecoat or colorcoat is next applied over the primer. A typical basecoat or colorcoat comprises a pigment, which may include metallic flakes in the case of a metallic finish. In order to protect and preserve the aesthetic qualities of the finish on the vehicle, it is well known to provide a clear (unpigmented) topcoat over the colored (pigmented) basecoat, so that the basecoat remains unaffected even on prolonged exposure to the environment or weathering.
Coating compositions comprise one or more film-forming polymers. Most commonly, acrylic polymers are linear in structure and cure, upon application, by reaction with crosslinking agents. However, the use of non-linear graft copolymers has been disclosed. For example, U.S. Pat. No. 4,801,653 to Das et al. describes the use of hydroxy functional graft copolymers. Das et al. disclose grafting by a condensation reaction between epoxy groups of a glycidyl ester, contained in an acrylic polymer, and carboxy groups on at least a portion of vinyl monomers which are polymerized in the presence of the acrylic polymer.
In preparing graft polymers in general, various living polymerization methods have been disclosed for obtaining functional ended polymers by selective termination of living ends. Such functionally ended polymers may subsequently be attached to another polymer, that is, as so-called macromonomer "arms" on a polymeric backbone to form a comb or graft copolymer. Webster, in "Living Polymerization Methods,"251 SCIENCE 887 (Feb. 22, 1991) generally discloses living polymerization methods for preparing architectural forms of polymers, including graft and comb copolymers.
U.S. Pat. No. 4,680,352 to Janowicz et al., U.S. Pat. No. 4,722,984 to Janowicz, and PCT WO 87/03605 disclose the use of cobalt (Co) chelates as chain transfer agents in free radical polymerization. The latter patents disclose that macromonomers prepared by cobalt chain transfer can be polymerized to produce graft copolymers which are useful in coating and molding resins, including high solid finishes and aqueous or solvent based finishes. The use of such polymers, however, have so far found only limited use in the automotive finishes area, as for example disclosed in U.S. Pat. No. 5,010,140.
The present invention relates to aqueous coating compositions. The evolution of environmental regulations has led to the need for products with lower volatile organic content (VOC). However, it is far from trivial to develop aqueous products with desirable properties for automotive finishes. As mentioned above, such finishes must be high performance in terms of aesthetic qualities and durability.
Water dispersible polymers are well known in the art and have been used to form waterbased coating compositions, pigment dispersions, adhesives and the like. Graft copolymers containing carboxyl groups and the preparation of these polymers is shown in Japanese Laid Open Patent Application (Kokai) No. 1-182304 dated Jul. 20, 1989. This reference shows graft copolymers that have carboxyl groups and discloses side chains from acrylic and methacrylic acid that have hydrophilic properties. This reference further teaches the use tertiary alcohol-based ester units of acrylic or methacrylic acid to form a macromonomer which is used to form a graft copolymer and then is hydrolyzed to form carboxylic-acid groups on the polymer. The process taught by the reference is an inefficient process which does not form pure graft copolymer but results in a mixture of graft copolymer and low molecular weight components that are detrimental to pigment dispersions formed from the graft copolymer and finishes formed from such a composition.
BASF EP 0363723 describes an acid-functional acrylic copolymer dispersion for use in an original equipment manufacturer (OEM) clear coat to be crosslinked with a melamine formaldehyde binder. The acrylic copolymer is prepared in a solvent in a two-stage process where the hydrophilic part (acid-functional monomer) is concentrated in one of the two stages. The overall copolymer is afterwards neutralized with an amine and dispersed in water. The difference between a one stage product is the solids/viscosity relation being most favorable for the two stage acrylic. A disadvantage of this technology is the fact that the hydrophilic part needs to be over 60% of acid functional monomer which could give problems in humidity resistance. The present method has the advantage that acid or amine functional copolymer macromonomers could be used which provide advantages in terms of humidity resistance, appearance, and lower minimum film-forming temperatures. Also, little to no cosolvent is needed to prepare the graft copolymer dispersion. Another disadvantage is that introducing hydroxy functional monomers in the hydrophilic part has strong negative effects on the solids/viscosity balance. Finally, another disadvantage is that the two stage acrylic needs to be prepared in cosolvents.
Bayer patents EP 0218906 and EP 0324334 describe the synthesis of hydroxy-acid functional acrylic copolymers prepared in solution before neutralizing with an amine and dispersing in water. This has the disadvantage of the solids/viscosity balance referred to above.
Bayer EP 0334032 describes the synthesis of an acid-functional urethane oligomer which is used to stabilize a waterborne acrylic copolymer dispersion. This technology does not allow hydroxy-functional groups (for crosslinking) in the hydrophilic stabilizing part.
AKZO U.S. Pat. No. 5,098,947 describes urethane modified acrylic copolymer dispersions for waterborne coatings. This technology is also limited by the use of cosolvents in which the urethane part is prepared.
As indicated above, the aqueous finishes disclosed in the prior art have significant disadvantages, for example, in terms of humidity resistance, acid resistance, durability, appearance and other properties. Thus, the problem of developing aqueous finishes with improved properties remains and has been the subject of considerable research and development in the automotive coatings industry.
The present invention offers significant advantages. Acid or amine functional macromonomers can be used which provide improved humidity resistance and appearance. Lower minimum film-forming temperatures may be used. Little to no cosolvent is needed to prepare the graft copolymer dispersion. The acid-functional or amine-functional graft can also be hydroxy functional for crosslinking with the amino formaldehyde and/or the (un)blocked polyisocyanate crosslinkers.